How is the word "Himmaleh" used in this Emily inson poem?

That's most likely an alternative spelling of "Himalaya." (I'm sure you're familiar with the mive Himalaya mountain range in Asia.) I don't know if some other spelling was common in inson's time, or if "Himmaleh" is her own personal quirky version of the word. The poem seems to be saying that strong people will collapse and become weak if you try to "Give Balm" to them -- that is, if you try to make things easy for them -- but if you give them an enormous challenge, if you give them a tremendous burden to bear, if you metaphorically pile a whole huge mountain range on their shoulders, they will find the strength to do what they have to do.